Earth's inner core is more like a ball of butter than a solid metal sphere, a new study suggests

• Scientists have long thought Earth's inner core is like a huge solid metal ball. 
• But an experiment revealed it may be a lot softer — more like butter, a lead scientist said. 
• It could help explain why Earth's magnetic field is so weird.

A new study challenges the long-held assumption that Earth's inner core is hard, like a solid metal sphere.